1. Field of the Invention
The present invention relates to a photovoltaic module, a photovoltaic module array, a photovoltaic system (photovoltaic power generation apparatus), and a method of detecting failure of a photovoltaic module.
2. Related Background Art
With the recent spreading use of photovoltaic modules, there is a rapidly growing demand for photovoltaic modules suitable for use in medium-scale electric power systems installed outdoors, particularly in the personal houses. Generally, in the case of using the photovoltaic module for electric power generation, a plurality of the photovoltaic modules are connected in series (hereinafter, referred to as “string”) for generating a voltage not smaller than a certain value, and a plurality of the strings are connected in parallel to form a photovoltaic module array.
FIG. 29 is a circuit diagram showing the constitution of a conventional photovoltaic module. In FIG. 29, there are shown a photovoltaic module 1, a photovoltaics 1a, and a bypass diode 1b connected parallel to the photovoltaics 1a. When a shadow is partly formed on the photovoltaic module 1 thereby increasing the electrical resistance of the photovoltaics 1a and causing the application of the voltage generated in other modules within the string as an inverse bias to the module 1 (hereinafter, referred to as “partial shadow”), the bypass diode 1b prevents the application of an inverse bias to the photovoltaics 1a in the photovoltaic module 1 thereby preventing the damage of the photovoltaic cell. Also, when a photovoltaic module which was part of the photovoltaic module array exhibited abnormal output, it was usually necessary, for detecting the position of the failure, to check whether the electrical output is normal in each string, then to interrupt the operation of the photovoltaics-photovoltaic system and to measure the electrical output of each photovoltaic module constituting the string by utilizing the output terminals of each photovoltaic module.
However, since the output terminals are usually provided on the back surface (a surface opposite to a light incident surface) of the photovoltaic module, it was very difficult to locate a failure position by using the output terminals after the installation of the photovoltaic module. On the other hand, if terminals for inspection are provided in each of the modules in such a way that they are exposed to the exterior, they may cause leakage of electricity or danger of electrical shock, thereby causing reliability problems. For these reasons, terminals for inspection were not provided. Consequently, for locating the failed photovoltaic module in a photovoltaic module array, the current flowed in the wiring of the photovoltaic module array is typically measured utilizing a clamping ampere meter.
It is preferable in practical use to have a photovoltaic module provided with current detecting not exposed to exterior in order to locate a failed position. An example of such a photovoltaic module includes the photovoltaic module shown in FIGS. 30A and 30B, as disclosed in the Japanese Patent Application Laid-Open No. 6-125105 and the photovoltaic module shown in FIG. 31, as disclosed in the Japanese Patent Application Laid-Open No. 9-148613. In FIGS. 30A and 30B, reference characters 1c and 1d indicate magnetic field generating means, and in FIG. 31, reference character 1e indicates light emitting means. In the photovoltaic module shown in FIG. 30A, a current flows in a bypass diode 1b to generate a magnetic field by the magnetic field generating means 1c. The photovoltaic module shown in FIG. 30B is so constructed that by the electromotive force generated by the photovoltaics 1a, an operation current flows into the magnetic field generating means 1d to generate a magnetic field. The photovoltaic module shown in FIG. 31 is so constructed that a current flows into the bypass diode 1b to turn on the light emitting means 1e. The current detecting means shown in FIG. 30B utilizes an operation current c1 generated during the operation of the photovoltaics 1a, while the current detecting means shown in FIGS. 30A and 31 utilize a current c2 flowing into the bypass diode 1b when the voltage of photovoltaics 1a is lowered.
However, the conventional method of detecting failure of a photovoltaic module array has the following problems. First, when the photovoltaics fails, a current flowing into the bypass diode is generated only when the failure of the photovoltaics is an open circuit failure; this method cannot be applied to short circuit failure. Second, the failure may not be detected in some cases, depending on the configuration of the photovoltaic module array. This problem becomes more conspicuous when the photovoltaic module array is equipped with a blocking diode for blocking a reverse current.
An open circuit failure means a failure such as an open circuit of the photovoltaic cell itself constituting the photovoltaics, or, in the case of plural photovoltaic cells constituting the photovoltaics, a failure such as breaking of a wiring connecting such photovoltaic cells.
Also, a short circuit failure means a failure such as a short circuit of the photovoltaic cell itself constituting the photovoltaics (including partial short circuit of the photovoltaic cell itself, the same is applied hereinafter), or, in the case of plural photovoltaic cells constituting the photovoltaics, a failure such as the short circuit of the wiring connecting such photovoltaic cells.
Both the open circuit failure and the short circuit failure are modes of failure, and these failure states (modes) are referred to as the open circuit failure mode and the short circuit failure mode, respectively.
The above two problems will be further explained with reference to FIGS. 32A to 34B. FIGS. 32A and 32B show the cases of a short circuit failure of the photovoltaics 1a in the photovoltaic module. In such a situation, regardless of the failure state, a current 2 flows into the failure signal generating means D1 for detecting the failure by the “absence” of the operation current of the photovoltaics 1a. On the other hand, even in the case of a failure state, the current 2 does not flow into the failure signal generating means D2 for detecting the failure by the “presence” of the current flowing into the bypass diode 1b. Distinguishing the failure state from the normal state is not possible in either case, and the failed photovoltaic module in the string cannot be detected. FIGS. 33A and 33B show examples of the photovoltaic module array constituted by connecting plural strings in parallel. FIG. 33A shows the state in the normal operation, while FIG. 33B shows the state in the failure state. Reference character 3 indicates a photovoltaic module array, and 3c indicates blocking diodes for preventing loss resulting from the reverse current generated in the case of generating a voltage difference. In such a configuration, when a photovoltaic module 1A constituting a part of a string 3a fails and reaches an open state, the operation current 2 does not flow through the string 3a at all, as shown in FIG. 33B, if the sum of open circuit voltages Voc2+Voc3 generated in other photovoltaic modules 1B and 1C is lower than the sum of operation voltages V4+V5+V6 of the string 3b. In other words, there is obtained a state that a current does not flow into all the bypass diodes and the photovoltaics constituting the string 3a. Consequently, it is not possible to detect the failed photovoltaic module in the string 3a. 
FIGS. 34A and 34B are circuit diagrams showing an example of the photovoltaic module array formed by connecting in series a plurality of parallel members 3e of the photovoltaics. FIG. 34A shows the state in the normal operation, while FIG. 34B is an equivalent circuit diagram showing a failure state. In FIG. 34A, I1+I2 indicates the operation current 2 in a normal operation. Even when a photovoltaic module 1D is damaged to generate an open circuit failure as shown in FIG. 34B, if the load current I1′+12′ is not larger than the operation current of the non-failed photovoltaic module 1E, reverse bias is not applied to the failed photovoltaic module 1D at all, so that no current flows into the bypass diode in such module. Consequently, the failure cannot be detected by the type means of detecting the failure by the current flowing into the bypass diode.
It is possible to locate a failure point by detecting the operation current of the photovoltaics by employing the constitution as shown in FIG. 30B. However, since the signal generating means Id such as the magnetic generating means is connected in series to the photovoltaics, the almost amount of the operation current of the photovoltaics flowed into the signal generating means almost the all amount of the operation current of the signal generating means was not small, and this loss becomes a large value which is not negligible, particularly in a large-area photovoltaic module of a large current.